84 Boating New Zealand
A common problem, especially
for boats moored on a marina
berth, is stray current.
This is electricity ‘leaking’ into
the surrounding water, which
may not even originate from
your boat. Left unchecked this
can cause rapid corrosion of
any metal components, with
pitted holes especially common
in aluminium hulls.
Douglas says specialist
equipment can measure the
extent of the problem at your
marina and an appropriate
solution can be implemented.
Using the correct anodes is, of
course, critical, while copper
earthing plates fitted to the hull
is another way of protecting
your boat.STRAY CURRENT
Any boat bigger than a small runabout will probably
carry multiple batteries, with one or more batteries
dedicated to starting the engine, and a second unit to
run the onboard electrics.
This installation prevents cell damage, as there are
two types of lead-acid batteries: starting, and deep-
cycle. Starting batteries deliver a large current for a
short period and cannot withstand repeated discharge.
A deep-cycle battery provides sustained power over an
extended period without being damaged.
For two diferent battery types to co-exist happily
in the boat, a voltage sensing relay (VSR) is used. This
ensures the starting battery is charged up first by the
engine’s alternator, and only once it’s fully topped-up
is the house battery charged. It also prevents the house
circuit from draining power from the starting battery- so the starting battery remains fully charged.
Added to this complexity is new battery technology,
which requires higher charging voltages than in the past.
Most engine alternators deliver a peak of around 14.2 volts,
but modern AGM and calcium batteries may require 14.4 to
14.6 volts to charge properly.
Douglas recommends replacing the built-in
regulator with an external regulator to get optimum
charging and extend battery life. He says this will save
money in the long term.
BATTERIES, CHARGING AND ISOLATING